Charge Recombination Deceleration by Lateral Transfer of Electrons in Dye-Sensitized NiO Photocathode.

Control of charge separation and recombination is critical for dye-sensitized solar cells and photoelectrochemical cells, and for p-type cells, the latter process limits their photovoltaic performance. We speculated that the lateral electron hopping between dyes on a p-type semiconductor surface can effectively separate electrons and holes in space and retard recombination. Thus, device designs where lateral electron hopping is promoted can lead to enhanced cell performance. Herein, we present an indirect proof by involving a second dye to monitor the effect of electron hopping after hole injection into the semiconductor. In mesoporous NiO films sensitized with peryleneimide (PMI) or naphthalene diimide (NDI) dyes, dye excitation led to ultrafast hole injection into NiO from either excited PMI* (τ < 200 fs) or NDI* (τ = 1.2 ps). In cosensitized films, surface electron transfer from PMI- to NDI was rapid (τ = 24 ps). Interestingly, the subsequent charge recombination (ps-µs) with NiO holes was much slower when NDI- was generated by electron transfer from PMI- than when NDI was excited directly. We therefore indicate that the charge recombination is slowed down after the charge hopping from the original PMI sites to the NDI sites. The experimental results supported our hypothesis and revealed important information on the charge carrier kinetics for the dye-sensitized NiO photoelectrode system.

Highly Efficient and Stable Pure Two-Dimensional Perovskite-Based Solar Cells with the 3-Aminopropionitrile Organic Cation.

Pure two-dimensional (2D) perovskite (n = 1)-based perovskite solar cells (PSCs) have been proven to have excellent stability against humidity, but the photovoltaic performance is very poor due to the parallel orientation to the substrate and mismatched energy alignment in the PSC device. We report herein a novel bulky organic cation of 3-aminopropionitrile (3-APN) for constructing a pure 2D hybrid lead-iodide perovskite. The crystal structure of (3-APN)2PbI4 features a stable layered and undistorted PbI6 octahedral geometry (∠Pb-I-Pb = 180°) with a small I···I distance (4.66 Å), and the crystals grow in a dominant out-of-plane direction to the substrate. In addition, the existence of an intramolecular H bond between cyano groups and ammonium heads result in an appropriate valence band level of (3-APN)2PbI4 for a well-matched energy level alignment in the device, benefitting the interfacial charge transfer and hence a better photovoltaic performance. As a result, the PSC with the pure 2D (3-APN)2PbI4 perovskite-based PSC achieves a power conversion efficiency of 3.39%, which is the highest value thus far for the pure 2D lead-iodide perovskite family, to the best of our knowledge. More importantly, this pure 2D (3-APN)2PbI4 perovskite-based PSC demonstrates excellent stability against humidity. This work demonstrates that there is great potential to realize efficient and stable pure 2D perovskite-based PSCs through the wise design of organic cations.

Self-Assembled Ionic Liquid for Highly Efficient Electron Transport Layer-Free Perovskite Solar Cells.

Electron transport layer (ETL)-free perovskite solar cells (PSCs) are attractive because they have fewer layers and hence are lower in cost, but their inferior photovoltaic performance, as compared to ETL-containing PSCs, greatly restricts their practical application. This study concerns the design and synthesis of a hydroxyethyl-functionalized imidazolium iodide ionic liquid, the determination of its single crystal structure, and its self-assembly on a conductive substrate for ETL-free PSCs. The self-assembly of the ionic liquid on the conductive substrate is found to lower the work function of the conductive substrate and enhance interfacial electron extraction while retarding interfacial charge recombination. As a consequence, the power conversion efficiency is improved remarkably from 9.01 % to 17.31 % upon self-assembly of the ionic liquid on the conductive substrate. This finding provides a new way to assemble highly efficient ETL-free PSCs.

4-(Aminoethyl)pyridine as a Bifunctional Spacer Cation for Efficient and Stable 2D Ruddlesden-Popper Perovskite Solar Cells.

Two-dimensional (2D) Ruddlesden-Popper (RP) perovskites are attractive due to their appealing environmental stability. We demonstrate herein a spacer cation, 4-(aminoethyl)pyridine (4-AEP), for preparation of 2D RP perovskite films. The 4-AEP can not only act as a spacer cation but also coordinate with the Pb2+ ions in PbI2 with the nitrogen atom on the pyridine ring. High-quality 2D RP perovskite films can thus be formed as the coordination interaction retards the crystallization rate of the 2D RP perovskites. As a result, the solar cell employing the (4-AEP)2MAn-1PbnI3n+1 (n = 5) 2D perovskite achieves a power conversion efficiency (PCE) of 11.68% with good air stability, which is much better than the phenylethylamine spacer cation at the same conditions (PCE = 7.95%). This work provides a new idea for designing novel spacer cations toward efficient and stable 2D RP perovskite solar cells.

Pyridine-Terminated Conjugated Organic Molecules as an Interfacial Hole Transfer Bridge for NiOx-Based Perovskite Solar Cells.

To engineer the NiOx/perovskite interface and promote interfacial hole transfer, two pyridine-terminated conjugated small organic molecules (PTZ-1 and PTZ-2) are synthesized to link the NiOx and perovskite layers for NiOx-based perovskite solar cells (PSCs). One terminal pyridine group interacts with the NiOx layer, while the other one coordinates with the Pb atoms of the perovskite layer, erecting an interfacial hole transfer bridge between NiOx and perovskite. Surface modification of the NiOx film with the PTZ molecules is able to enhance hole extraction, increase hole mobility and conductivity of NiOx, reduce defect density, and retard interfacial charge recombination. As a consequence, power conversion efficiency is improved from 12.53 to 16.25 and 17.00% upon surface modifications of NiOx with PTZ-1 and PTZ-2, respectively. Furthermore, the modified PSCs exhibit almost no hysteresis and show good stability after storage in air (relative humidity of 30-40%) for 500 h without encapsulation.

Core-Sheath Paraffin-Wax-Loaded Nanofibers by Electrospinning for Heat Storage.

Paraffin wax (PW) is widely used for smart thermoregulation materials due to its good thermal performance. However, the leakage and low thermal conductivity of PW hinder its application in the heat storage field. Accordingly, developing effective methods to address these issues is of great importance. In this study, we explored a facile approach to obtain PW-loaded core-sheath structured flexible nanofibers films via coaxial electrospinning technique. The PW as the core layer was successfully encapsulated by the sheath-layer poly(methyl methacrylate). The diameter of the fiber core increased from 395 to 848 nm as the core solution speed rate increased from 0.1 to 0.5 mL/h. In addition, it can be seen that higher core solution speed rate could lead to higher PW encapsulation efficiency according to the transmission electron microscopy results. The core-sheath nanofiber films, moreover, possessed the highest latent heat of 58.25 J/g and solidifying enthalpy of -56.49 J/g. In addition, we found that after 200 thermal cycles, there was little change in latent heat, which demonstrated that it is beneficial for the PW-loaded core-sheath structure to overcome the leakage issue and enhance thermal stability properties for the thermoregulation film.

A simple and low-cost combustion method to prepare monoclinic VO2 with superior thermochromic properties.

In this approach, the VO2 nanoparticles have been successfully fabricated via combusting the low-cost precursor solution consisted of NH4VO3, C2H6O2 and C2H5OH. By the XRD, TEM and XPS analysis, it can be found that the synthetic monoclinic VO2 is single crystal and no impurity is defined. After dispersing the VO2 nanoparticles into the polymer, the solar modulation of VO2-based composite film is up to 12.5% with luminous transmission and haze around 62.2% and 0.5%, respectively. In other words, the composite films show high performance of thermochromic properties. This could open an efficient way to fabricate low-cost and large-scale VO2 (M) nanoparticles and thermochromic films.

Shape-controlled synthesis and influence of W doping and oxygen nonstoichiometry on the phase transition of VO2.

Monoclinic VO2(M) in nanostructure is a prototype material for interpreting correlation effects in solids with fully reversible phase transition and for the advanced applications to smart devices. Here, we report a facile one-step hydrothermal method for the controlled growth of single crystalline VO2(M/R) nanorods. Through tuning the hydrothermal temperature, duration of the hydrothermal time and W-doped level, single crystalline VO2(M/R) nanorods with controlled aspect ratio can be synthesized in large quantities, and the crucial parameter for the shape-controlled synthesis is the W-doped content. The dopant greatly promotes the preferential growth of (110) to form pure phase VO2(R) nanorods with high aspect ratio for the W-doped level = 2.0 at% sample. The shape-controlled process of VO2(M/R) nanorods upon W-doping are systematically studied. Moreover, the phase transition temperature (Tc) of VO2 depending on oxygen nonstoichiometry is investigated in detail.